Over the last decade, the advent of solid-state lighting has led to rapid advances in the production of high brightness Light Emitting Diode (LED). LED's brightness is now competing with incandescent and fluorescent light sources mainly due to breakthroughs on chip structures, improved extraction and thermal management of LED lighting system.
The light extraction efficiency reflects the ability of photons emitted inside the LED chip to escape into the surrounding medium. For example, the index of refraction of Gallium phosphide-based materials is close to 3.4, compared with 1 for air and 1.5 for epoxy. This results in a critical angle of 17° for air and 25° in epoxy, respectively. If a single interface is considered only 2% of the incident light into air and 4% into epoxy will be extracted. As a comparison, the index of refraction of Gallium nitride-based materials is close to 2.3. This results in a critical angle of 26° into air and 41° into epoxy. If a single interface is considered only 5% of the incident light into air and 12% into epoxy will be extracted. The rest is reflected into the semiconductor where it will eventually be reabsorbed or recycled and results in the performance degradation of the device.
While light extraction efficiency is an important consideration in the design of LEDs, other factors may also be important. For example, to ensure that the entire active layer in the LED is utilized in light emission, it is desirable to spread the electrical current to the entire active layer. To enhance the efficient use of electrical current in light generation, the ohmic contact resistance with the LED should also be as low as possible. To enhance light extraction, the layers between the active layer and the emitting surface of the LED should have high light transmission characteristics. In addition, in order to efficiently reflect light generated by the active layer traveling in directions away from the light emitting surface of the LED, the different layers of the light reflector employed should have high index contrast.
One type of reflectors for LEDs is proposed in the paper “Omni-Directional Reflectors for Light-Emitting Diodes,” by Jong Qyu Kim, et al. Proc. of SPIE Volume 6134, pages D-1 to D-12, 2006. In FIG. 5 of this paper, a GaInN LED with an omni-directional reflector (ODR) is shown. This LED structure comprises a sapphire substrate supporting a GaInN LED. A thin layer of oxidized Ruthenium (Ru) is used as a semi-transparent low-resistance p-type ohmic contact. A quarter-wave thick silicon dioxide low-refractive index layer perforated by an array of silver micro-contacts and a thick silver layer are also employed. In section 3.3.3 on page D-9 of this paper, however, the authors Kim, et al. indicated that the above structure of FIG. 5 is disadvantageous because the above design “needs absorptive semi-transparent current spreading layer, such as RuO2, . . . , which leads to a decrease in reflectivity of the ODR.” Furthermore, the refractive index of silicon dioxide is deemed to be not low enough for high refractive index contrast with high-index semiconductor materials, which limits further improvement of light extraction efficiency in GaN-based LEDs.
As an alternative, the authors proposed an ODR structure illustrated in FIGS. 11 and 12 of the paper. In this alternative ODR structure, the oxidized Ruthenium and silicon dioxide layers in FIG. 5 are replaced by an indium-tin oxide (ITO) nanorod low index layer illustrated in FIG. 12 of the paper. However, as illustrated in FIG. 13 of the paper, the ITO nanorod layer provides mediocre ohmic contact characteristics. Moreover, the ITO material reacts strongly with metal, such as silver. When the ITO nanorod layer proposed by Kim, et al. comes into contact with a silver substrate underneath, interdiffusion occurs at the interface which greatly reduces the reflective properties of the resulting structure. This will also greatly reduce the light extraction efficiency of the LED. It is therefore desirable to provide an improved LED structure in which the above-described difficulties are alleviated.
Thermal management has always been a key aspect of the proper use of LEDs. Poor thermal management leads to performance degradation and reduced lifetime of LEDs.
A substrate of high thermal conductivity becomes a necessity for the operation of high power LEDs. It allows heat generated at the chip level to be transferred efficiently away from the chip through the substrate. Given that conventional red (AlGaInP) and blue (InGaN) LED is grown from N+ GaAs and sapphire substrates, respectively, one of the major drawbacks of GaAs and sapphire are their poor thermal conductivity. GaAs and sapphire have thermal conductivity values of 50, and 40 w/m K, respectively. Obviously, replacing GaAs or sapphire with a carrier of high thermal conductivity such as one made of Si (150 W/m□K), Cu (400 W/m□K), or AlSiC (180 W/mK) can significantly improve the LED performance through better heat dissipation.
The substrate from which the LED is grown is referred to herein as the growth substrate. The high thermal conductive substrate from which the LED is transferred to is referred to herein as the substitute substrate.
To create good bonding with growth substrate, it is necessary to have a substitute substrate with CTE closely match with that of growth substrate. In the case of GaN based LED grown from sapphire substrate, the fabrication process introduces considerable compressive stress on the LED due to the slightly higher CTE of sapphire than GaN. When the sapphire substrates are replaced with substitute substrate, the LED may be damaged if such compressive force is released rapidly. It is well known that GaN based LED materials are strong under compression and weak under tensile force. Therefore, it is desirable to preserve the compressive force on the LED to enhance the reliability of LED chips for subsequent thermal processes such as laser lift-off and die bonding. As a result, it is desirable to use a substrate material that has CTE equal to or slightly greater than sapphire substrate (˜6 ppm/K) to replace the sapphire growth substrate.
Metal matrix composites are well known material that typically includes a discontinuous particulate reinforcement phase within a continuous metal phase. An example is silicon carbide reinforced aluminum matrix composite, AlSiC, which is made by infiltrating porous silicon carbide with molten aluminum. The AlSiC metal matrix composite system has the positive attributes of high thermal conductivity, low and tailorable coefficient of thermal expansion and high strength. These attributes render AlSiC suitable as a substitute substrate.
Bonding material is another important part of the LED system, since the bonding phase will not only play an important part to the thermal management system but also need to survive the subsequent chip processing processes such as laser lift-off, and die bonding processes. Ideally, the bonding material has high thermal conductivity, high strength, high temperature stability and is low cost. In the case of GaN/sapphire system, the LED growth wafer usually is not smooth, partly due to the inherent stress between sapphire and GaN and partly due to the particulate contamination in the growth process. To fully cover the non-smooth surface of growth wafer, solder bonding method is the preferred method. Among various solder bonding materials, Sn based soft solders are relative cheap but have low strength and low temperature stability; hard solders such as AuSn or AuGe have high strength, high temperature stability but are high cost and have relatively low thermal conductivity. It is therefore desirable to develop a better bonding material that does not have the above shortcomings.
To bond a substitute substrate to a growth substrate, it is necessary to provide good contact between the bonding wafers. In the case of GaN-based LEDs grown from sapphire substrates, the GaN growth process introduces considerable stress on the LED due to the CTE mismatch between sapphire and GaN material. As shown on FIG. 5, the stress created ˜30 um curvature from center to the edge of the GaN/sapphire growth wafer. Obviously, it is difficult to generate good bonding between a flat substitute wafer and a curved growth wafer. Poor bonding between growth substrate and substitute substrate will not only result in lower yield during the sapphire removal process but also create reliability problem on LED chips. Commercial wafer bonding devices and techniques are designed to handle flat wafers, not wafers with curved surfaces. It is therefore desirable to develop a better wafer-bonding device/process to overcome the unique bonding problem.